The term IHSS, an acronym for Idiopathic Hypertrophic Subaortic Stenosis, is an older designation for a serious heart condition. It has largely been replaced by the more accurate name, Hypertrophic Cardiomyopathy (HCM). This shift reflects a deeper scientific understanding of the disease, which is characterized by an abnormal thickening of the heart muscle. This article explains the condition, its causes, presentation, and current management strategies.
Understanding IHSS and Hypertrophic Cardiomyopathy
IHSS described a pathology where the heart muscle, specifically the wall separating the two lower chambers (the ventricle septum), becomes excessively thick. The “subaortic stenosis” implied this thickening always created a physical blockage, restricting blood flow out of the left ventricle and into the aorta. This restriction, known as Left Ventricular Outflow Tract Obstruction, forces the heart to work much harder to pump blood.
The medical community transitioned to the umbrella term Hypertrophic Cardiomyopathy because the original name was misleading. Current data show that approximately one-third of patients with muscle thickening do not experience a significant outflow tract obstruction either at rest or during physical activity. The modern classification focuses on the muscle disorder itself, noting the specific diagnosis as either obstructive or non-obstructive HCM.
HCM is defined by the thickening of the left ventricular wall that cannot be explained by other cardiac or systemic diseases. The core issue is diastolic dysfunction, a problem with the heart’s ability to relax and fill with blood, even if the pumping function remains preserved. The thickened, stiff muscle tissue is less compliant, leading to elevated pressures inside the heart’s chambers and contributing to symptoms. The historical term “idiopathic,” meaning of unknown cause, is also less accurate due to advances in genetic research identifying the disorder’s molecular origins.
The Genetic Basis of the Condition
Hypertrophic Cardiomyopathy is recognized as the most common inherited heart muscle disorder. The vast majority of cases are caused by mutations in genes that provide instructions for making the heart’s contractile proteins, collectively known as the sarcomere. These microscopic units are responsible for the heart’s ability to contract and relax with each beat.
The two genes most frequently implicated in the condition are MYH7 and MYBPC3, which together account for 60 to 70 percent of identified pathogenic variants. Mutations in these genes disrupt the normal function of the sarcomere, leading to cellular stress and ultimately triggering the massive growth of heart muscle cells. This genetic inheritance pattern typically follows an autosomal dominant model, meaning only one copy of the mutated gene is needed to potentially cause the disease.
The condition exhibits variable penetrance, which explains why one family member with a specific mutation may develop severe symptoms while another with the same mutation remains relatively unaffected. Genetic testing plays a significant role in identifying the specific mutation within a family. Identifying the underlying genetic cause allows for proactive monitoring and risk stratification in individuals who have not yet shown physical signs of the disease.
Recognizing Common Symptoms
The clinical presentation of Hypertrophic Cardiomyopathy is highly variable, ranging from no symptoms at all to severe manifestations. The most common complaint is dyspnea, or shortness of breath, particularly during physical exertion. This shortness of breath is often a consequence of the stiffened left ventricle causing blood to back up into the lungs.
Patients may also experience chest pain, medically termed angina, even without blockages in the coronary arteries. The thickened heart muscle requires more oxygen than the blood supply can deliver, especially during periods of increased demand, leading to this pain. Palpitations, described as a fluttering or pounding sensation in the chest, are another frequent symptom that can indicate underlying electrical rhythm disturbances.
Syncope (temporary loss of consciousness) or presyncope (a near-fainting sensation) may occur during or immediately after physical activity. These events are concerning because they signal inadequate blood flow to the brain, often caused by the outflow obstruction or a life-threatening arrhythmia. Any symptoms, particularly when they worsen with increased activity, should prompt an immediate medical evaluation.
Diagnostic Procedures and Management Strategies
Diagnosis begins with a comprehensive medical history, focusing on symptoms and any family history of the condition or sudden cardiac death. Initial non-invasive testing includes an Electrocardiogram (ECG) to assess the heart’s electrical activity, which often shows abnormalities related to the increased muscle mass. The definitive diagnosis relies on imaging to directly visualize the heart’s structure and function.
The Echocardiogram (Echo) is the primary diagnostic tool, using sound waves to create live images of the heart chambers and muscle thickness. An echo measures the degree of hypertrophy and determines if a Left Ventricular Outflow Tract Obstruction is present. Cardiac Magnetic Resonance Imaging (MRI) provides detailed, high-resolution images of the muscle structure and identifies areas of scarring. A stress test can be used to provoke an outflow gradient not evident at rest, helping to guide treatment decisions.
Management strategies are tailored to the presence of symptoms and the individual’s risk of sudden cardiac death. Pharmacological treatment often begins with medications like beta-blockers or calcium channel blockers, which slow the heart rate and allow the stiff ventricle more time to fill with blood. Newer therapies, such as mavacamten, work directly by reducing the excessive contractility of the sarcomere, specifically for patients with obstructive HCM.
For patients with significant, drug-resistant outflow obstruction, interventional procedures are considered to reduce the muscle thickness. These include Septal Myectomy, a surgical procedure where a portion of the thickened septum is removed to widen the outflow tract, and Alcohol Septal Ablation, where alcohol is injected into a coronary artery branch to intentionally shrink the obstructing tissue. The most serious management consideration is the prevention of sudden cardiac death, often achieved by implanting an Implantable Cardioverter-Defibrillator (ICD) in high-risk patients. This small device continuously monitors the heart rhythm and delivers an electrical shock to restore a normal rhythm if a life-threatening arrhythmia occurs.